In a conventional marine outboard motor, a propeller is driven by a drive output of the motor to propel a boat through the water. Most large outboard motors of this type inject the exhaust under water to reduce engine noise and increase propulsive thrust. The exhaust generated by the motor flows downwardly through an exhaust channel and exits the motor through the propeller. This type of motor is referred to as an exhaust-through-the-hub (ETH) motor.
When the drive output rotates the propeller for forward motion, the exhaust is discharged downstream of the propeller. In contrast, when the drive output rotates the propeller for reverse motion, the exhaust is discharged such that it can be entrained into the propeller. Even though the exhaust intrudes into the water stream being moved by the propeller in reverse motion, a high reverse thrust level is possible since the propeller is not surrounded by a housing.
Another type of conventional outboard motor has a pump jet driven by the drive output. In a pump jet, an impeller or rotor is mounted directly to the drive output in place of the propeller, and a ducted housing surrounds the rotor. Modifications to the gear case, cooling or sealing components of the motor are typically not required for a pump jet. Benefits of a pump jet include reducing hazards to swimmers in the vicinity of the motor, protecting the rotor from interference with and damage by foreign objects in the water, and improving the efficiency and performance of the motor. Another benefit inherent with a pump jet is a greater steering response based upon a directed jet of water resulting therefrom.
As with a propeller, when the drive output rotates the rotor for forward motion, the exhaust is discharged downstream of the rotor. Unfortunately, in reverse motion, the exhaust may enter the water stream within the housing and little or no reverse level thrust is achieved. The Applicants provided one approach to this problem, as disclosed in U.S. Pat. No. 5,325,662.
In the '662 patent, exhaust from a power unit 11 flows downwardly through an exhaust channel 12 and through the rotor hub 55 into an exhaust plenum 44, as illustrated in FIG. 1. At least one hollow stator vane 50a extends radially from the exhaust plenum 44, and the exhaust is discharged through the stator vane. Since the exhaust is radially discharged outwardly through an exhaust port 64 in the wall of a stator housing 28, the exhaust will not enter the water stream when the pump jet 40 is in reverse motion.
Due to the practical need to discharge large volumes of exhaust at wide open throttle, a plurality of hollow stator vanes 50a, 50b are required. The pump jet area blockage associated with a plurality of stator vanes 50a, 50b directly competes with the available internal water flow area required by the pump jet 40 to produce acceptable thrust levels. Exhaust systems relying on radial exhaust discharge through hollow stator vanes are difficult to design and fabricate with acceptable propulsive performance.